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Phase Stability and Morphology of Gel Grown Crystals: The Case of CdCl2-bpp Polymeric System

1
Dipartimento di Chimica, Università degli Studi di Milano, via Golgi 19, I-20133 Milano, Italy
2
ISTM-CNR, Via Golgi 19, 20133 Milano, Italy
3
Centre for Materials Crystallography, Aarhus University, Langelandsgade 140, DK-8000 Aarhus, Denmark
4
Dipartimento di Scienza dei Materiali, Università degli Studi di Milano Bicocca, via Cozzi 55, I-20125 Milano, Italy
*
Author to whom correspondence should be addressed.
Crystals 2019, 9(7), 363; https://doi.org/10.3390/cryst9070363
Received: 30 May 2019 / Revised: 25 June 2019 / Accepted: 12 July 2019 / Published: 16 July 2019
(This article belongs to the Special Issue Crystal Growth in Gels)
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Abstract

A phenomenological study is carried out on a complex two-component diffusion-reacting system in gel, that is, the Cd-1,3-bis(4-pyridyl)propane (Cd-bpp) coordination polymer. The latter can exist in three solid forms, which exploit a 1:1 correspondence among the Cd/bpp ratio, the crystal structure and the crystal morphology (1/2: bipyramids; 2/3: needles; 1/3: plates). The aim was to clarify the role of key physicochemical variables (reactant concentrations, composition of the solvent and density of the transport medium) in determining the chemical nature and the morphology of the final crystallization products. The gel method was tested in a variety of different crystallization configurations, including single and double diffusion techniques. The density of the gel primarily affects the morphology of the synthesized crystals, with denser media favouring the needle-like 2/3 Cd-bpp species and diluted ones the 1/2 Cd-bpp bipyramidal one. However, higher densities of the gel are generally associated to strained crystals. The solvent composition is also important, as for example the 1/2 Cd-bpp bipyramids require at least a minimum amount of ethanol to appear. We demonstrated that in gel the strict “equality” stoichiometric criteria for metal-to-ligand ratios can be sometimes eluded, as non–equilibrium concentrations can be locally attained. In this respect, the crystallization geometry was proven to act as a key tool to influence the crystallization output, as it determines the direction and magnitude of the concentration gradients. Finally, the use of U tubes to perform one-pot screenings of a large part of the crystallization space is discussed. View Full-Text
Keywords: coordination polymers; crystal growth; gel growth technique; crystal morphology coordination polymers; crystal growth; gel growth technique; crystal morphology
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).

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Lo Presti, L.; Moret, M.; Rizzato, S. Phase Stability and Morphology of Gel Grown Crystals: The Case of CdCl2-bpp Polymeric System. Crystals 2019, 9, 363.

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